Narrow Wells Research Articles

(Invited) Deep Ultraviolet Light Emitting Diodes: Physics, Performance, and Applications

The key factor for developing deep UV LEDs is material’s quality of nitride epitaxial films. It could be dramatically improved by using novel MEMOCVD® technique. Using very narrow quantum wells (QWs) and energy band gap engineering in deep UV LEDs improves overlap between electron and hole wave functions and increases the Internal Quantum Efficiency (IQE) and External Quantum Efficiency (EQE). A novel transparent and reflecting top contact further boosts IQE and EQE and increases the power output and high external quantum efficiency of our Deep UV LEDs. Research is now under way to use these devices for improving quality and extending storage time of fruits and vegetables. Other applications include water and air purification, sterilization, biological threat identification, applications in medicine, biology, industrial processes, defense, and homeland security.

Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO3quantum wells

The electrical and structural characteristics of SmTiO3/SrTiO3/SmTiO3 and GdTiO3/SrTiO3/GdTiO3 heterostructures are compared. Both types of structures contain narrow SrTiO3 quantum wells, which accommodate a confined, high-density electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO3 quantum wells embedded in GdTiO3 show a metal-to-insulator transition when their thickness is reduced so that they contain only two SrO layers. In contrast, quantum wells embedded in SmTiO3 remain metallic down to a single SrO layer thickness. Symmetry-lowering structural distortions, measured by quantifying the Sr-column displacements, are present in the insulating quantum wells, but are either absent or very weak in all metallic quantum wells, independent of whether they are embedded in SmTiO3 or in GdTiO3. We discuss the role of orthorhombic distortions, orbital ordering, and strong electron correlations in the transition to the insulating state.

Increasing the ν = 5/2 gap energy: an analysis of MBE growth parameters

The fractional quantized Hall state at the filling factor ν = 5/2 is of special interest due to its possible application for quantum computing. Here we report on the optimization of growth parameters that allowed us to produce two-dimensional electron gases (2DEGs) with a 5/2 gap energy up to 135 mK. We concentrated on optimizing the molecular beam epitaxy (MBE) growth to provide high 5/2 gap energies in ‘as-grown’ samples, without the need to enhance the 2DEGs properties by illumination or gating techniques. Our findings allow us to analyse the impact of doping in narrow quantum wells with respect to conventional DX-doping in AlxGa1−xAs. The impact of the setback distance between doping layer and 2DEG was investigated as well. Additionally, we found a considerable increase in gap energy by reducing the amount of background impurities. To this end growth techniques like temperature reductions for substrate and effusion cells and the reduction of the Al mole fraction in the 2DEG region were applied.

Photoluminescence and exciton resonances over the scattered light in multiphonon spectra of resonant scattering in the CdTe/ZnTe superlattices with narrow quantum wells

Photoluminescence and Raman scattering spectra in CdTe/ZnTe heterostructures and superlattices with narrow quantum wells (4.8–9.2 A) in a temperature range of 5–300 K have been measured. The temperature dependences of the intensity of exciton luminescence in isolated quantum wells have been studied, and the thermal activation energies associated with the effective barriers for electrons and holes have been determined. In CdTe/ZnTe heterostructures, the binding energies of an exciton with a heavy hole have been determined as functions of the quantum well width. The multiphonon Raman spectra that exhibit distinctive features, such as the weak intensity of nLO phonon lines of ZnTe (n 2), and the multiple participation in scattering of acoustic LA phonons with large wave vector, have been investigated. The results have been explained based on the concept of the relaxation of hot excitons over the exciton band.

Low-threshold lasing action in an asymmetric double ZnO/ZnMgO quantum well structure

ZnO/Zn0.85Mg0.15O asymmetric double quantum well (ADQW) and multiple quantum well (MQW) were fabricated with plasma assisted molecular epitaxy on c-plane sapphire, with their optical properties and optical pumped lasing characteristics studied. Due to the good crystalline quality, the lasing threshold of the MQW is ∼20 kW cm−2. The widths of the narrow well (NW) and the wide well (WW) of the ADQW were chosen to fascinate rapid LO phonon assisted carrier tunneling from NW to WW, so as to enhance the exciton density at the WW. Very low lasing threshold of 6 kW cm−2 has been achieved.

Diffusion and deformations in heterosystems with GaN/AlN superlattices, according to data from EXAFS spectroscopy

Multilayered samples with extremely narrow GaN quantum wells in an AlN host are synthesized via ammonia MBE. The parameters of the microstructure are determined by means of EXAFS spectroscopy, high-resolution electron microscopy, and low-angle scattering. Their relationship to the morphology of GaN/AlN superlattices is established. The influence of growth conditions and the thickness of superlattices on their optical properties and mixing in the near-boundary layers is established.

Electron g-factor in GaAs/AlGaAs quantum wells of different width and barrier Al concentrations

Using electrically detected electron spin resonance technique, three components of electron g-factor tensor are studied in a number of asymmetrically doped GaAs/AlGaAs quantum wells grown in the [001] direction. The investigated quantum wells are characterized by different well widths and barrier Al concentrations. The dependence of gzz component along the growth direction on the heterostructure band gap was shown to be universal. The band gap was measured with the aid of photoluminescence. The two-fold in-plane g-factor anisotropy with the [110] and [11̄0] principal axes turned out to vanish in narrow quantum wells. Three nonzero components of â tensor of the linear in the magnetic field corrections to the g-factor are measured for a number of wells of different parameters. The â tensor components are strongly dependent on the well width.

151Eu Mössbauer study of multiferroic Eu0.75Y0.25MnO3

We are herewith reporting the 151Eu Mossbauer spectra collected on a polycrystalline powder sample of Eu0.75Y0.25MnO3 from 15 K to room temperature. All the spectra consist of a single line, whose shape and related sample thickness are dependent on the temperature T. The thermal trend of the mean square displacement of Eu ion, obtained from the spectra analysis, clearly reveals a large low-temperature anharmonicity and in concomitance with the onset of the magnetic ordering consists in a linear strong decrease interrupted by two narrow wells at 29.5 K and 40 K. This behavior is interpreted in connection with the transfer of spectral weight from the 120 cm-1 optical phonon to the electromagnonic modes. The T-trend of the central shift shows that Eu3+ electronic ground state in the magnetically ordered phase differs from the one in the paramagnetic state. Finally, the temperature dependence of the hyperfine field under T N gives a contribution to interpret some controversial features regarding the phase-diagram.

Generation of potential wells used for quantum codes transmission via a TDMA network communication system

ABSTRACTThis paper proposes a technique of quantum code generation using optical tweezers. This technique uses a microring resonator made of nonlinear fibre optics to generate the desired results, which are applicable to Internet security and quantum network cryptography. A modified add/drop interferometer system called PANDA is proposed, which consists of a centred ring resonator connected to smaller ring resonators on the left side. To form the multifunction operations of the PANDA system—for instance, to control, tune and amplify—an additional Gaussian pulse is introduced into the add port of the system. The optical tweezers generated by the dark soliton propagating inside the PANDA ring resonator system are in the form of potential wells. Potential well output can be connected to the quantum signal processing system, which consists of a transmitter and a receiver. The transmitter is used to generate high‐capacity quantum codes within the system, whereas the receiver detects encoded signals known as quantum bits. Therefore, an entangled photon pair can be generated and propagated via an optical communication link such as a time division multiple access system. Here, narrower potential wells with a full‐width half‐maximum of 3.58 and 9.57 nm are generated at the through and drop ports of the PANDA ring resonator system, respectively, where the amplification of the signals occurs during propagation inside the system. Copyright © 2013 John Wiley & Sons, Ltd.

Microscopic theory for Doppler velocimetry of spin propagation in semiconductor quantum wells

We provide a microscopic theory for the Doppler velocimetry of spin propagation in the presence of spatial inhomogeneity, driving electric field, and the spin orbit coupling in semiconductor quantum wells in a wide range of temperature regime based on the kinetic spin Bloch equation. It is analytically shown that under an applied electric field, the spin density wave gains a time-dependent phase shift $\ensuremath{\phi}(t)$. Without the spin-orbit coupling, the phase shift increases linearly with time and is equivalent to a normal Doppler shift in optical measurements. Due to the joint effect of spin-orbit coupling and the applied electric field, the phase shift behaves differently at the early and the later stages. At the early stage, the phase shifts are the same with or without the spin-orbit coupling. While at the later stage, the phase shift deviates from the normal Doppler one when the spin-orbit coupling is present. The crossover time from the early normal Doppler behavior to the anomalous one at the later stage is inversely proportional to the spin diffusion coefficient, wave vector of the spin density wave, and the spin-orbit coupling strength. In the high temperature regime, the crossover time becomes large as a result of the decreased spin diffusion coefficient. The analytical results capture all the quantitative features of the experimental results, while the full numerical calculations agree quantitatively well with the experimental data obtained from the Doppler velocimetry of spin propagation [Nat. Phys. 8, 153 (2012).]. We further predict that the coherent spin precession, originally thought to be broken down at high temperature, is robust up to the room temperature for narrow quantum wells. We point out that one has to carry out the experiments longer to see the effect of the coherent spin precession at higher temperature due to the larger crossover time.

Measuring Thermal Rupture Force Distributions from an Ensemble of Trajectories

Rupture, bond breaking, or extraction from a deep and narrow potential well requires considerable force while producing minimal displacement. In thermally fluctuating systems, there is not a single force required to achieve rupture, but a spectrum, as thermal forces can both augment and inhibit the bond breaking. We demonstrate measurement and interpretation of the distribution of rupture forces between pairs of colloidal particles bonded via the van der Waals attraction. The otherwise irreversible bond is broken by pulling the particles apart with optical tweezers. We show that an ensemble of the particle trajectories before, during and after the rupture event may be used to produce a high fidelity description of the distribution of rupture forces. This analysis is equally suitable for describing rupture forces in molecular and biomolecular contexts with a number of measurement techniques.

Ground state terahertz quantum cascade lasers

A terahertz quantum cascade laser (THz QCL) architecture is presented in which only the ground state subbands of each quantum well are involved in the transport and lasing transition. Compared to state-of-the art THz QCLs based on the resonant-phonon scheme, ground state QCLs employ narrower wells so that all high-energy subbands are pushed up far above the occupied subband levels, significantly reducing parasitic interactions. Data on the experimental realization of two types of ground state QCLs are presented, in which the result of lasing above 5 THz is demonstrated.

Binding energies and oscillator strengths of impurity states in wurtzite InGaN/GaN staggered quantum wells

Using the variational methods, we have calculated the binding energies of the lowest donor states, 1s and 2p±, in wurtzite InGaN/GaN staggered quantum wells. The binding energies in narrow wells are larger in magnitude than the values in bulk GaN due to the quantum confinement effects. However, the energies decrease sharply in wider wells because of the weakening confinement due to the strong built-in electric field inside the well. The binding energies of donors placed at the opposite edges of the well are quite different as the built-in electric field forms an asymmetric, triangular potential inside the well. The oscillator strength of the possible transitions between the donor states is then computed by modelling them as the states of a two-level atom. A magnetic field applied along the growth direction splits up the degenerate 2p± states. The amount of splitting in the quantum well is found to be small possibly due to the heavy electron effective mass inside the well. The oscillator strength of the transition between the donor states becomes greater with the increasing magnetic field.

Nano‐constriction device for rapid protein preconcentration in physiological media through a balance of electrokinetic forces

We describe a methodology to steeply enhance streptavidin protein preconcentration within physiological media over that achieved by negative dielectrophoresis (NDEP) through utilizing a DC offset to the AC field at nanoscale constriction gap devices. Within devices containing approximately 50-nm constriction gaps, we find that the addition of a critical DC field offset (1.5 V/cm) to the NDEP condition (∼200 V(pp) /cm at 1 MHz) results in an exponentially enhanced extent of protein depletion across the device to cause a rapid and steeply rising degree of protein preconcentration. Under these conditions, an elliptical-shaped protein depletion zone that is extended along the device centerline axis forms instantaneously around the constrictions to result in protein preconcentration along the constriction sidewall direction. Through a potential energy diagram to describe the electrokinetic force balance across the device, we find that the potential energy barrier due to NDEP is gradually tilted upon addition of DC fields, to cause successively steeper potential wells along the sidewall direction for devices containing smaller constriction gaps. Hence, for approximately 50-nm constriction gaps at a critical DC field, the ensuing narrow and deep potential energy wells enable steep protein preconcentration, due to depletion over an exponentially enhanced extent across the device.

Design of GaAs/Al x Ga1−x As asymmetric quantum wells for THz-wave by difference frequency generation

The energy levels, wave functions and the second-order nonlinear susceptibilities are calculated in GaAs/Al0.2Ga0.8As/Al0.5Ga0.5As asymmetric quantum well (AQW) by using an asymmetric model based on the parabolic and non-parabolic band. The influence of non-parabolicity can not be neglected when analyzing the phenomena in narrow quantum wells and in higher lying subband edges in wider wells. The numerical results show that under double resonance (DR) conditions, the secondorder difference frequency generation (DFG) and optical rectification (OR) generation susceptibilities in the AQW reach 2.5019 μm/V and 13.208 μm/V, respectively, which are much larger than those of the bulk GaAs. Besides, we calculate the absorption coefficient of AQW and find out the two pump wavelengths correspond to the maximum absorption, so appropriate pump beams must be selected to generate terahertz (THz) radiation by DFG.

Relativistic fermion in a spherically symmetric potential well of finite depth in a two-dimensional space

The problem of existence of bounded relativistic fermion states in a spherically symmetric well of finite depth in a two-dimensional space is investigated. The well depth critical for the appearance of standard states with energies E = m, 0, and –m is determined; moreover, cases with zero and nonzero fermion momenta are considered. Approximate analytical expressions for the critical depths of narrow and wide wells are derived which are in good agreement with the results of numerical calculations. Approximate energies of levels located on the boundaries of the upper and lower continuums and determined analytically are in good agreement with the results of numerical calculations.

Eigenstate localization in an asymmetric coupled quantum well pair

Optical pumping of a type-I/type-II coupled asymmetric quantum well pair induces a spatially separated two dimensional charge carriers plasma in the well’s wide and narrow parts. Treating the two coupled wells as a single system we find that the eigenstate probability distribution localizes exclusively either in the wide or the narrow parts of the well pair. The energy of the narrow-well localized state determines the minimal excitation energy for optically pumped charge carriers separation. In a previously used design [Guliamov et al., PRB 64 035314 (2001)] this narrow well transition energy was measured to correspond to a wavelength of 646nm. We propose modifications to the design suggested earlier with the purpose of pushing up the energy required for the optical pumping of the two-dimensional plasma into the green and blue regions of the visible spectrum.

Full-Field Heavy-Oil Conversion From Cold to Hot Production

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 146695, ’Full-Field Heavy-Oil Conversion From Cold to Hot Production: Challenges and Solutions,’ by F.E. Jansen, SPE, A. Curtis, SPE, L.V. Mejia-Cana, J. Ramsdal, SPE, and J.O. Selboe, SPE, Statoil ASA, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The paper has not been peer reviewed. With an increasing interest in extra-heavy-oil (EHO) extraction, several heavy-oil fields will be produced initially by a cold-production scheme, and, at a later stage, thermal production in the form of steam injection can be used to enhance the recovery. In an EHO cold-to-hot conversion scheme, several choices must be made with respect to well positioning, completion options, timing, and scale of the conversion. Introduction Challenges in establishing a good reservoir-management plan for EHO reservoirs over their entire life span stem from the complexity of modeling heavy-oil recovery, fine-grid requirements, very large models, and dedicated tools (i.e. thermal vs. conventional simulators). In this sense, crude oils can be divided roughly into three groups: those that flow without added heat (e.g., conventional oils), those that do not flow without added heat (bitumen), and those that will flow initially but yield a less-than-satisfying recovery with only cold flow (e.g., many EHO fields that are the focus of this paper). The challenge for reservoir-management optimization of EHO reservoirs is combining slow pressure propagation (low mobility), unfavorable mobility ratio to water, and the large area, thus requiring a very large number of wells to achieve good reservoir drainage and acceptable field-production rates. The environmental requirements regarding minimization of footprint and water handling, combined with possible conversion from cold to hot production, make the handling of wells even more complex. Cold Production If the in-situ viscosity is low enough to allow economical natural flow without added heating of the reservoir, primary cold production normally is preferred if the reservoir compressibility and aquifer support are high enough to deliver a reasonable recovery (often by use of horizontal wells, downhole diluent injection, pumps, and narrow well spacing to increase well rates and field recovery). In some cases, the reservoir and mobility relationship is appropriate for waterflooding. Several cold-production options exist: waterflooding, with or without polymer, and cold heavy-oil production with sand (CHOPS). However, full-life-cycle evaluations must be made because waterflood and CHOPS preclude subsequent hot-production conversion. Important for all of these techniques is the use of downhole pumps—hydraulic pumps, electrical submersible pumps, and progressing-cavity pumps (PCPs). The most common is the PCP because of its reliability and ability to handle high-viscosity fluid and sand. Diluent, in the form of light hydrocarbons, often is injected downhole (as deep into the well as possible) to reduce the effective viscosity in the well and flowlines. Diluent handling and possible regeneration can be important.

Quantum-correlated two-photon transitions to excitons in semiconductor quantum wells

The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers.

Oscillatory acoustic phonon relaxation of excitons in quantum dot molecules

We study electrically tunable self-assembled InAs quantum dot molecules through photoluminescence (PL) and time-resolved PL measurements. For the model we assume quantum dots with cylindrical symmetry, for which the confinement potentials have been modeled as narrow quantum wells in the growth and in-plane directions matched to parabolic potentials. We focus on the hole scattering rates by bulk acoustic phonons, as these rates are the leading contribution for the neutral indirect exciton relaxation rate when the electron localizes primarily on one dot. The hole–phonon scattering structure factor for acoustic phonons is found to contain a phase relationship between the phonon wave and the hole wave function, which can be tuned by an external electric field. The phase relationship leads to interference effects and tunable oscillatory relaxation rates of indirect excitons, in agreement with experiments.